The invention relates to an apparatus and a method for measuring the thickness of a foil, a film or a material layer by means of a measuring head which is spaced away from the film and with which a capacitive thickness measurement is performed.
Methods of measuring film thickness are used in particular in the production of plastic films, so that the film thickness and, as the case may be, the evenness of the film thickness over the width of the film may be monitored during the manufacturing process and may be regulated, if necessary. For example, in a blow film apparatus, the measuring head is arranged at the film bubble, which is inflated by internal blow air and is drawn-off upwardly, in such a manner that it may revolve around the film bubble and thus can measure the film thickness on the entire periphery of the film bubble during one revolution. As it would be difficult, in particular in case of blow films, to arrange components of the measuring head on both sides of the film or to provide a metallic support or backup roller on the back side of the film, the measuring head should be constructed such that the measurement can be performed from one side of the blow film.
Measurement methods are known, which do not fulfill this feature. Thus, DE 199 59 570 A1 discloses a sensor for contactless measurement of materials, wherein the object to be measured is guided on a support, and a ball guided in a cage rolls over the object with a little application force. A height displacement of the ball within the cage is measured by laser triangulation, while, simultaneously, the distance of the cage from the metallic support is measured by means of an eddy current sensor. Adding the measured results gives the thickness of the object. Disadvantageous is on the one hand the necessity of a metallic support and, on the other hand, the contact of the ball with the object to be measured.
Further, the company Micro Epsilon commercializes measuring devices in which, also, an eddy current measurement is performed for determining the spacing of a sensor from a metallic support roller, on which the film is disposed, while an additional measurement of the spacing between the film surface and the sensor is performed by means of a laser micrometer, or a capacitive measurement of the film thickness is performed, depending on the type of the device. In the capacitive measurement, which is highly sensitive to distance variations, the distance value obtained by the eddy current measurement is used for compensating distance fluctuations. Disadvantageous is in any case the necessity of a metallic support roller. In another known method for measuring thicknesses of profiled materials, respective laser-optical triangulation sensors for detecting the surface of the material are disposed on both sides of a measuring gap. The thickness of the material passed through the measuring gap is determined from the measured results, the width of the measuring gap being additionally monitored by means of an eddy current measurement. Here, it is a disadvantage that a triangulation sensor is necessary on both sides of the material.
DE 36 12 914 A1 discloses a method of measuring the thickness of paper, wherein an air cushion provides a constant spacing between a surface of the paper, which is disposed on a support, and a measuring head, while, simultaneously, the spacing between the measuring head and the support is determined by means of an inductive displacement sensor. Again, it is particularly the necessity of a support which is disadvantageous. Moreover, the accuracy of the measurement is limited, due to fluctuations in the thickness of the air cushion.
A capacitive method, wherein a contactless thickness measurement is performed from one side of a film while the measuring head is held spaced away from the film by means of an air cushion, is known from EP 0 591 239 and GB 1 436 698. Therein, an air cushion is created between the measuring head and the film, for example by supplying gas under pressure, and the measuring head is held in an approximately constant distance from the surface by the air cushion. This prevents a contact between the film and the measuring head.
A single-sided capacitive thickness measurement utilizes the effect, that the electrically non-conductive film acts as a dielectric in a fringe field of a plate capacitor. Thus, a change in film thickness corresponds to a change in capacity of the capacitor. A problem is that a change in the distance between the film and the capacitive measurement device tampers the capacitance measurement and, consequently, the thickness measurement, wherein the measurement error may be of the same order of magnitude as the measured film thickness. This disturbing influence of distance changes becomes larger with increasing distance between the measuring head and the film, since the field strength of the capacitor decreases strongly at larger distances. In the production of blow films, however, the movements of the film inevitably lead to fluctuations in the distance between the measuring head and the film. These fluctuations disturb the capacitive measurement and lead to erroneous measurement results
DE 195 11 939 discloses a sensor head, which is supported without friction in a mount of a sensor. Therein, gas is supplied via a gas supply opening at the measuring head, and the gas will flow away laterally between the measuring head and the film, so that an equilibrium between the repelling action of the gas pressure and the suction action of the flowing gas is established. The measuring head is thereby held at an essentially constant distance from the film. In addition to this fine-adjustment of the distance, an automatic coarse-adjustment of the position of the measuring head is proposed, by means of a distance sensor which may be an optical, ultrasonic or also a capacitive sensor, for example. This measuring head has the drawback that, on the one hand, it is required that the capacitive thickness measurement shall be as insensitive as possible to the distance within the available distance range. On the other hand, the frictionless suspension of the measuring head is very complex. Moreover, the sensor head will not always follow-up to the film with sufficiently quick response, because of its mass of inertia, so that the admissible distance range may be left.
EP 0 801 290 discloses a method, wherein an air cushion is also created between the measuring head and the film. An active feedback control of the distance between the measuring head and the film is performed indirectly by measuring and controlling the pressure within the air cushion between the measuring head and the film. The pressure is controlled, for being adjusted to a reference pressure, by shifting the position of the measuring head by means of a linear motor. In addition to or in place of the pressure control, a correction of the distance-dependent results of the capacitive thickness measurement shall be performed on the basis of the distance data that have been obtained by the pressure measurements. To this end, a simple functional relation between the magnitude of the measurement error and the distance between the measuring head and the film is assumed. According to this, the thickness d(x) measured at the distance x is related to the thickness d0 measured at the distance x=0 by d(x)=d0/(1+x). A drawback is high effort for the pressure measurement, which, moreover, permits to derive the distance between the measuring head and the film only indirectly and with a low reliability and depends on the advance speed, the curvature and the surface properties of the film. The position adjustment of the measuring head influences the pressure in the air cushion only indirectly and with time delay and, moreover, the mass of inertia of the measuring head makes it difficult to adjust the position of the measuring head quickly and exactly. Under certain operating conditions of a blow film apparatus, it may therefore not be possible to stably maintain the target reference pressure.